The condensation of 1-O-acetyl-2,3,5-tri-O-benzoyl-beta-D-ribofuranose (X) with 1,2,4-triazole-3-carboxylic acid methyl ester (VII) by means of PNPP at 160 C C gives 1-(2,3,5-tri-O-benzoyl-beta-D-ribofuranosyl)-1,2,4-triazole-3-carboxylic acid methyl ester (XI), which is finally treated with NH4OH in methanol.
The silylation of 1,2,4-triazole-3-carboxylic acid methyl ester (VII) with hexamethyldisylazane (HMDS) gives 1-(trimethylsilyl)-1,2,4-triazole-3-carboxylic acid methyl ester (XII), which is condensed with 2,3,5-tri-O-benzoyl-beta-D-ribofuranosyl bromide (XIII) by means of PNPP at 160 C to afford the previously described intermediate 1-(2,3,5-tri-O-benzoyl-beta-D-ribofuranosyl)-1,2,4-triazole-3-carboxylic acid methyl ester (XI).
The condensation of 1,2,3,5-tetra-O-acetyl-beta-D-ribofuranose (XIV) with 1,2,4-triazole-3-carboxylic acid methyl ester (VII) by means of PNPP at 160 C gives 1-(2,3,5-tri-O-acetyl-beta-D-ribofuranosyl)-1,2,4-triazole-3-carboxylic acid methyl ester (XV), which is finally treated with NH4OH in methanol. The condensation of 2,3,5-tri-O-acetyl-beta-D-ribofuranosyl bromide (XVI) with 1-(trimethylsilyl)-1,2,4-triazole-3-carboxylic acid methyl ester (XII) in acetonitrile gives the previously reported intermediate 1-(2,3,5-tri-O-acetyl-beta-D-ribofuranosyl)-1,2,4-triazole-3-carboxylic acid methyl ester (XV).
Viramidine can be synthesized by several related ways: 1) Reaction of triethyl orthoformate (I) with 1-cyanoformimidic acid hydrazide (II) by means of anhydrous HCl in dioxane gives 1,2,4-triazole-3-carbonitrile (III), which is condensed with tetra-O-acetyl-?D-ribofuranose (IV) by means of di-p-nitrophenyl phosphate (BNPP) at 150 oC to yield a mixture of the regioisomeric ribonucleosides (V) and (VI), easily separated by chromatography. Finally, the suitable regiosomer 3-cyano-1-(2,3,5-tri-O-acetyl-?D-ribofuranosyl)-1,2,4-triazole (V) is treated with anhydrous NH3 and NH4Cl at 85 oC in a pressure bomb (1, 2). 2) Alternatively, condensation of 2,3,5-tri-O-acetyl-?D-ribofuranosyl bromide (VII) with 1-(trimethylsilyl)-1,2,4-triazole-3-carbonitrile (VIII) - prepared by treatment of triazole (III) with hexamethyldisilazane in dry acetonitrile - in acetonitrile also gives the regioisomeric mixture of nucleosides (V) and (VI) (1). 3) Acylation of ribavirin (1-?D-ribofuranosyl-1,2,4-triazole-3-carboxamide) (IX) with Ac2O and pyridine gives the triacetate (X), which is dehydrated by means of POCl3 and triethylamine in chloroform to yield 3-cyano-1-(2,3,5-tri-O-acetyl-?D-ribofuranosyl)-1,2,4-triazole (V). Treatment of nucleoside (V) with NaOMe in methanol affords the methyl carboximidate (XI), which is finally treated with NH4Cl and dry NH3 gas in methanol (3).